Electrode assemblies and associated fixation members for implantable medical devices

ABSTRACT

A fixation member of an electrode assembly for an implantable medical device includes a tissue engaging portion extending along a circular path, between a piercing distal tip thereof and a fixed end of the member. The circular path extends around a longitudinal axis of the assembly. A helical structure of the assembly, which includes an electrode surface formed thereon and a piercing distal tip, also extends around the longitudinal axis and is located within a perimeter of the circular path. The tissue engaging portion of the fixation member extends from the distal tip thereof in a direction along the circular path that is the same as that in which the helical structure extends from the distal tip thereof. The electrode assembly may include a pair of the fixation members, wherein each tissue engaging portion may extend approximately one half turn along the circular path.

TECHNICAL FIELD

The present invention pertains to implantable medical devices, and, morespecifically, to electrode assemblies thereof.

BACKGROUND

Many electrode assemblies of implantable medical devices, for thedelivery of stimulation therapy and/or for diagnostic sensing, includefixation members configured to hold an electrode surface of the assemblyin stable and intimate contact with tissue at an implant site. Thoseskilled in the art are familiar with electrode assemblies that include ahelical structure; the helical structure typically includes a piercingdistal tip to engage with tissue so that the helical structure may bescrewed into the tissue and thereby fix an electrode surface of theassembly in intimate contact with the tissue. In some of these electrodeassemblies the electrode surface is formed directly on the helicalstructure. Although such a helical structure can provide adequatefixation in many types of electrode assemblies, there is a need for newtypes of electrode assemblies and associated fixation members.

SUMMARY

Embodiments of the present invention encompass implantable medicaldevices (e.g., cardiac pacemakers), electrode assemblies thereof, andfixation members for the electrode assemblies. According to someembodiments, a fixation member for an electrode assembly, includes atissue engaging portion that extends along a circular path, between apiercing distal tip thereof and a coupling portion, or a fixed end, ofthe member, which is configured to be fixedly attached to a distalportion of the device. The circular path extends around a longitudinalaxis of the fixation member and of the electrode assembly, and theelectrode assembly further includes a helical structure that has anelectrode surface formed thereon, and a piercing distal tip adjacentthereto; the helical structure is located within a perimeter of thecircular path and also extends around the longitudinal axis. The tissueengaging portion of the fixation member extends from the distal tipthereof in a direction along the circular path that is the same as thatin which the helical structure extends from the distal tip thereof.Thus, rotation of the distal portion of the device, when the piercingdistal tips of the helical structure and the fixation member arepositioned in proximity to a target implant site, causes both thehelical structure and the tissue engaging portion of the fixation memberto engage within tissue at the implant site.

An implantable medical device, according to some embodiments, comprisesa relatively compact, or miniature capsule in which a pulse generatorand power source are hermetically sealed, and to which the electrodeassembly is directly coupled. In some embodiments, an outer diameter ofthe helical structure is approximately 2 French, while an outer diameterdefined by the tissue engaging portion of the fixation member isapproximately 20 French; and the electrode surface formed on the helicalstructure is preferably formed for ‘high impedance’ pacing, for example,having a gross surface area of approximately 1 to 2 square millimetersand an increased microscopic surface area, for example, being formed asa titanium nitride (TiN) coating overlaying the helical structure.According to some embodiments, another electrode surface is formed onthe fixation member, for example, to form a bipolar pace-sense pair withthe electrode surface formed on the helical structure.

Electrode assemblies, according to some preferred embodiments, include apair of the above-described fixation members, for example, wherein thefixed ends thereof are located opposite one another, on either side ofthe helical structure. The tissue engaging portion of each fixationmember may extend between approximately one quarter of a turn andapproximately one turn along the circular path, preferably,approximately one half of a turn. The piercing distal tip of eachfixation member may be coplanar with that of the helical structure, orproximally offset therefrom.

In some embodiments, the coupling portion, or fixed end, of the fixationmember includes a post that is offset from, and extends approximatelyparallel with the longitudinal axis, and the tissue engaging portion ofthe fixation member includes a first segment and a second segment,wherein the first segment extends along the circular path and proximallyfrom the post, in a longitudinal direction, and the second segmentextends along the circular path and distally from the first segment tothe distal tip, in an opposite longitudinal direction. Thus, the tissueengaging portion, according to these embodiments of the fixation memberhas a contour that may be likened to a saddle shape, wherein a gapbetween the distal portion of the device and the tissue engaging portionof the fixation member narrows in between the piercing distal tipthereof and the post of the coupling portion to create a kind of lock onthe tissue with which the fixation member is engaged.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative of particular embodiments of thepresent invention and therefore do not limit the scope of the invention.The drawings are not to scale (unless so stated) and are intended foruse in conjunction with the explanations in the following detaileddescription. Embodiments will hereinafter be described in conjunctionwith the appended drawings wherein like numerals/letters denote likeelements, and:

FIG. 1 is a schematic diagram showing potential implant sites forembodiments of the present invention;

FIG. 2A is a perspective view of an implantable medical device,according to some embodiments;

FIG. 2B is an elevation view of a portion of an electrode assembly ofthe device, according to some embodiments;

FIG. 3 is a plan view of a distal portion of a delivery device engagingthe medical device, according to some methods;

FIG. 4A is a perspective view of a pair of fixation members, accordingto some embodiments;

FIG. 4B is an elevation view of the pair of fixation members; and

FIG. 4C is a cross-section view through section line A-A of FIG. 4B.

DETAILED DESCRIPTION

The following detailed description is exemplary in nature and is notintended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the following description providespractical examples, and those skilled in the art will recognize thatsome of the examples may have suitable alternatives.

FIG. 1 is a schematic diagram showing the interior of a right atrium RAand a right ventricle RV of a heart. FIG. 1 illustrates some potentialendocardial implant sites 102, 103 for implantable electrode assemblies,according to embodiments of the present invention, for example, likethat shown in FIG. 2A. It should be noted that any other suitableimplant site, either endocardial or epicardial, may be selected forelectrode assembly embodiments. FIG. 2A is a perspective view of animplantable medical device 200, wherein the electrode assembly thereofincludes a helical structure 210, which extends distally from a distalportion 201 of device 200, and around a longitudinal axis 2 of theelectrode assembly, and on which a first electrode surface is formed,and a ring structure 220, on which a second electrode surface is formed.The pair of electrode surfaces can provide bipolar pacing and sensing ateither of implant sites 102, 103 shown in FIG. 1. According to theillustrated embodiment, both pulse generator electronic circuitry and abattery power source of device 200 are contained within a relativelycompact, or miniature, hermetically sealed capsule 25 thereof, forexample, that has a length of approximately 2 to 2.5 centimeters and adiameter of approximately 20 French (6-7 millimeters). Capsule 25 ispreferably formed from a biocompatible and biostable metal such astitanium, which is overlaid with an insulative layer, for example,parylene, polyimide, medical grade polyurethane, Polyether ether ketone(PEEK), or silicone. Hermetic feedthroughs, such as any known to thoseskilled in the art, couple each electrode surface to the pulse generatorcircuitry contained within capsule 25, which is configured, according tomethods known to those skilled in art, to sense, via the electrodesurfaces, either atrial depolarization (i.e. P-waves), for example, fromimplant site 102, or ventricular depolarization (i.e. R-waves), forexample, from implant site 103, and to apply stimulation pulses to themyocardial tissue for cardiac pacing when necessary.

With further reference to FIG. 2A, helical structure 210 is preferablyconfigured for high impedance pacing from the electrode surface thereof;thus, according to an exemplary embodiment, helical structure 210 has anouter diameter of approximately 2 French (0.6-0.7 millimeters), and thegross surface area of the electrode surface is between approximately 1square millimeter and approximately 2 square millimeters. In someembodiments, the electrode surface is formed along an outer portion ofhelical structure 210, while an inner portion thereof is insulated.Those skilled in the art will appreciate that the high impedance of thiselectrode surface, resultant of the relatively small gross surface areathereof, provides relatively efficient pacing stimulation, for example,to maximize the life of the battery power source of device 200. Helicalstructure 210 may be formed from a platinum iridium wire or a tantalumwire, for example, having an outer diameter of between approximately0.005 inch (0.13 mm) and approximately 0.010 inch (0.25 mm); and theelectrode surface of helical structure 210 may be formed by a titaniumnitride (TiN) coating that increases the microscopic surface area of theelectrode surface for an enhanced interface with the myocardial tissue,for example, at either of implant sites 102, 103. Ring structure 220 maylikewise be formed of either platinum iridium or tantalum and have a TiNcoating that forms the second electrode surface, which may have asurface area of approximately 50 millimeters, according to someembodiments.

Intimate contact between the electrode surface of helical structure 210and the myocardial tissue is attained by engaging a piercing distal tip211 of helical structure 210 in the tissue and then rotating structure210 counter-clockwise, according to FIG. 2A, to screw structure 210 intothe tissue. In order to stabilize and maintain this intimate tissuecontact of the electrode surface of helical structure 210, the electrodeassembly of device 200 further includes one or a pair of fixationmembers 230.

FIG. 2A illustrates each fixation member 230 including a tissue engagingportion 233, which is terminated by a piercing distal tip 231, whereineach tissue engaging portion 233 extends from the distal tip 231thereof, along a circular path, which defines a perimeter in whichhelical structure 210 is located. With further reference to FIG. 2A,tissue engaging portion 233 of each fixation member 230 extends alongthe circular path toward a fixed end, which is coupled to distal portion201 of device 200, and which includes a post 235 extending distally fromdistal portion 201. Posts 235 are shown located opposite one another oneither side of helix structure 210, being radially offset from, andextending approximately parallel with, longitudinal axis 2. According tothe illustrated embodiment, the direction along the circular path, inwhich each tissue engaging portion 233 extends from the correspondingdistal tip 231, is the same as in which helical structure 210 extendsfrom piercing distal tip 211 thereof, which is clockwise according toFIG. 2A. Thus, according to those embodiments in which helical structure210 is fixedly attached to distal portion 201 of device 200, likefixation members 230, and when piercing distal tips 211 and 231 arepositioned in proximity to a target implant site (e.g., site 102 or 103)for engagement with myocardial tissue, device 200 as a whole may berotated, for example, in the counter-clockwise direction, according toFIG. 2A (i.e. per arrow F of FIGS. 2B and 3), to screw both helicalstructure 210 and fixation members 230 into the tissue. Although FIG. 2Aillustrates each tissue engaging portion 233 extending approximately onehalf of a turn along the circular path, in alternate embodiments eachportion 233 may extend between approximately one quarter of a turn toapproximately one full turn around the circular path.

FIG. 2B is an elevation view of a portion of the electrode assembly ofdevice 200 wherein a contour of each tissue engaging portion 233,according to some embodiments, as it extends along the circular path ismore clearly seen. FIG. 2B illustrates each tissue engaging portion 233including a first segment 233A, which extends along the circular pathand bends proximally from a distal end 41 of the corresponding post 235,toward distal portion 201 of device 200, and a second segment 233B,which extends along the circular path and distally from thecorresponding first segment 233A, away from distal portion 201.According to the illustrated embodiment, piercing distal tips 231 offixation members 230 and piercing distal tip 211 of helical structure210 (FIG. 2A) are coplanar with one another and spaced at a distance Tfrom distal portion 201, wherein distance T may be between approximately1 millimeter and approximately 3 millimeters. According to somepreferred embodiments, a pitch of helical structure 210 (e.g.,approximately 0.016 inch) is less than or equal to that defined bytissue engaging portion 233 between distal end 41 of post 235 andpiercing distal tip 231. According to some alternate embodiments, asillustrated by the broken lines in FIG. 2B, helical structure 210 mayextend beyond distal tips 231, so that distal tips 231 are proximallyoffset from distal tip 211 of helical structure 210, in which casehelical structure 210 may engage within tissue at the target implantsite ahead of the engagement of one or both of portions 233, dependingon the character, or structure of the surface of the tissue at the site,and an orientation of the electrode assembly relative to the surface.With further reference to FIG. 2B, in conjunction with FIGS. 4A-C, theabove described contour of each tissue engaging portion 233 may belikened to a saddle shape, wherein a gap G between distal portion 201and tissue engaging portion 233 narrows to a distance N, in betweenpiercing distal tip 231 and post 235, to create a kind of lock on thetissue with which portion 233 is engaged.

As was mentioned above, in those embodiments wherein both helicalstructure 210 and one or a pair of fixation members 230 are fixedlyattached to distal portion 201 of device 200, device 200 may be rotatedto engage helical structure 210 and fixation member(s) 230 in tissue ata target implant site. With reference to FIG. 3, and according to someimplant methods, a delivery device 300 may be configured with an outersheath 340 and a telescoping inner member 360 that is configured toengage around a proximal portion 202 (FIG. 2A) of device 200. Withreference back to FIG. 1, delivery device 300, with device 200 loadedinto a distal portion 341 of outer sheath 340, may be inserted into thevascular system of a patient, for example, via a percutaneous entrysite, and distal portion 341 passed into the heart, via an inferior venacava IVC or a superior vena cava SVC thereof. Once distal portion 341 islocated in proximity to a target implant site, for example, one of sites102, 103, inner member 360 may be advanced, relative to outer sheath340, to push device 200 out from a distal opening of distal portion 341and into contact with the site, and then inner member 360 may berotated, per arrow F, to fix device in tissue M at the target implantsite. With reference back to FIG. 2A, according to some embodiments,device 200 includes a notch 205, which is formed in proximal portion 202and configured for engagement with a screw-driver type stylet, whereinthe tool may replace or form a part of telescoping inner member 360 thatrotates device 200 per arrow F.

It should be noted that, according to some embodiments, an electrodesurface may be formed on one or both of fixation members 230 to functionas a bipolar pace-sense pair with the above-described electrode surfaceformed on helical structure 210. Such a pace-sense pair may beadvantageous in some instances, over the pace-sense pair formed with theelectrode surface of ring structure 220, for example, when device 200 isimplanted in the right atrium RA for pacing function based on P-wavesensing, since the closer spacing between the electrode surface ofhelical structure 210 and that of one or both of fixation members 230 isless susceptible to far-field R-wave sensing.

FIG. 4A is a perspective view of a pair 430 of fixation members 230,wherein pair 430 may be machined, or cut, from a tubular member,according to some embodiments. FIG. 4B is an elevation view of pair 430;and FIG. 4C is a cross-section view through section line A-A of FIG. 4B.FIGS. 4A-C illustrate a coupling portion of each fixation member 230including the aforementioned post 235 and a base segment 435, whichextends from a proximal end 42 of post 235 and along a circular path,which is offset from the above-described circular path along whichtissue engaging portions 233 extend. With reference back to FIGS. 2A-B,base segments 435 may be fixed and embedded within distal portion 201 ofdevice 200, and thus hidden from view in these Figures. It should benoted that, in alternate embodiments, base segments 435 may extend indifferent paths, either arcuate or relatively straight. But, accordingto the illustrated embodiment, because pair 430 of fixation members 230is cut from a tubular member, both circular paths (of tissue securingportions 233 and base segments 435) extend around a longitudinal axis 4of the tubular member. The tubular member from which pair 430 is cut maybe any suitable biocompatible and rigid material, preferably a metallicmaterial, for example, titanium or tantalum.

According to the illustrated embodiment, and with reference to FIG. 4B,in conjunction with FIG. 2B, it may be seen that a centralcross-section, per section line A-A (aligned on axis 4), divides firstsegment 233A of each tissue engaging portion 233 from the correspondingsecond segment 233B and, thus, coincides with the above-describednarrowing of gap G. FIGS. 4A-B further illustrates one of posts 235including an optional extension 45 that protrudes radially outward, awayfrom longitudinal axis 4. Extension 45 may serve as a tissue stop, whentissue engaging portions 233 are being screwed into tissue, for example,as described above.

FIG. 4C illustrates a rectangular cross-section of each base segment 435and of each tissue engaging portion 233, wherein, according to anexemplary embodiment, the cross section of each tissue engaging portion233 is approximately 0.005 inch by 0.010 inch, and may vary slightly insize to enhance a strength thereof. FIG. 4C further illustrates piercingdistal tip 231 of one of tissue engaging portions 233 extending overfirst segment 233A of the other of portions 233, and spaced aparttherefrom by a distance R. According to an exemplary embodiment distanceR is approximately 0.018 inch.

In the foregoing detailed description, the invention has been describedwith reference to specific embodiments. However, it may be appreciatedthat various modifications and changes can be made without departingfrom the scope of the invention as set forth in the appended claims. Forexample, although exemplary embodiments of electrode assemblies havebeen described in the context of device 200, the described electrodeassemblies may be incorporated by implantable medical electrical leadsthat are configured to couple with separate pulse generator devices andextend distally therefrom, for example, into chambers of a heart.

The invention claimed is:
 1. An implantable medical device comprising a pulse generator, a power source, an hermetically sealed capsule that contains the pulse generator and the power source, and an electrode assembly, the electrode assembly comprising a longitudinal axis, a helical structure coupled to the capsule and extending distally from the capsule and around the longitudinal axis, a first electrode surface formed on the helical structure, and a second electrode surface electrically isolated from the first electrode surface, the helical structure including a piercing distal tip located adjacent the first electrode surface; and an improvement to the device comprising at least one fixation member, wherein each fixation member comprises: a fixed end coupled to the capsule, the fixed end comprising a post that extends approximately parallel with the longitudinal axis, between the distal portion of the device and the tissue engaging portion of the corresponding fixation member; and a tissue engaging portion terminated by a piercing distal tip, each tissue engaging portion extending from the corresponding distal tip to the corresponding fixed end along a circular path, the helical structure being located within a perimeter of the circular path; and wherein each tissue engaging portion extends from the piercing distal tip thereof in a direction along the circular path that is the same as that in which the helical structure extends from the piercing distal tip thereof, either clockwise or counter-clockwise, and wherein the fixed end of each fixation member of the electrode assembly further comprises a base segment from which the corresponding post extends distally to the corresponding tissue engaging portion, the base segment of each fixation member extending along an arcuate path, around the longitudinal axis.
 2. The device of claim 1, wherein the tissue engaging portion of each fixation member of the electrode assembly extends between approximately one quarter of a turn and approximately one turn along the circular path.
 3. The device of claim 1, wherein the piercing distal tip of each fixation member of the electrode assembly is approximately coplanar with the piercing distal tip of the helical structure.
 4. The device of claim 1, wherein the piercing distal tip of each fixation member of the electrode assembly is proximally offset from the piercing distal tip of the helical structure.
 5. The device of claim 1, wherein the second electrode surface of the electrode assembly is formed on the fixation member of the electrode assembly.
 6. The device of claim 1, wherein the electrode assembly further comprises a ring structure that extends around a perimeter of the capsule, and the second electrode surface is formed on the ring structure.
 7. The device of claim 6, wherein the electrode assembly further comprises a third electrode surface formed on the fixation member thereof.
 8. The device of claim 1, wherein an outer diameter of the helical structure of the electrode assembly is approximately 2 French, and an outer diameter defined by the tissue engaging portion of each fixation member of the electrode assembly is approximately 20 French.
 9. The device of claim 1, wherein a gross surface area of the first electrode surface is between approximately 1 square millimeter and approximately 2 square millimeters.
 10. The device of claim 1, wherein the first electrode surface comprises a TiN coating that overlays the helical structure.
 11. An implantable medical device comprising a pulse generator, a power source, an hermetically sealed capsule that contains the pulse generator and the power source, and an electrode assembly, the electrode assembly comprising a longitudinal axis, a helical structure coupled to the capsule and extending distally from the capsule and around the longitudinal axis, a first electrode surface formed on the helical structure, and a second electrode surface electrically isolated from the first electrode surface, the helical structure including a piercing distal tip located adjacent the first electrode surface; and an improvement to the device comprising at least one fixation member, wherein each fixation member comprises: a fixed end coupled to the capsule, the fixed end comprising a post that extends approximately parallel with the longitudinal axis, between the distal portion of the device and the tissue engaging portion of the corresponding fixation member; and a tissue engaging portion terminated by a piercing distal tip, each tissue engaging portion extending from the corresponding distal tip to the corresponding fixed end along a circular path, the helical structure being located within a perimeter of the circular path; and wherein each tissue engaging portion extends from the piercing distal tip thereof in a direction along the circular path that is the same as that in which the helical structure extends from the piercing distal tip thereof, either clockwise or counter-clockwise, and wherein the tissue engaging portion of each fixation member of the electrode assembly includes a first segment and a second segment, each first segment extending along the circular path and bending proximally from the corresponding post, toward the capsule of the device, and each second segment extending along the circular path and distally from the corresponding first segment, away from the capsule of the device. 